How do I save a pose in 2.8?
Choose the tab at the top of the screen that says "Animation". This will put a narrow "Timeline" panel at the bottom. Change the Timeline panel to "Dope Sheet"
The "Dope Sheet" panel has another drop-down menu near the panel-changing menu you just accessed. From this drop-down menu, change it from "Dope Sheet" (or whatever it may be) to "Action Editor".
This next step assumes your model is rigged to its intended armature. Select that armature and press CTRL + TAB. This will put you in "Pose" mode and the armature will turn a light blue color. In the top-middle of the Action Editor panel, there is a button that says "+    New". Click that to create a new action. You can also rename it.
Now, This is important, click that little icon near that button that looks like a gray shield. The program does not indicate this (and they should have, sine at least 2.79) but this is the button that saves your animations. I am telling you to press it now because if you don't, you will start posing your character, save your scene, exit the program, and be startled to find it's gone when you return. You don't have to save the animation immediately, but you have to before you change Actions or close the program, so be in the habit of doing it immediately.